Ammonia capturing by CO2 product liquid in water wash liquid

ABSTRACT

A method for capturing ammonia present in combustion flue gas subjected to carbon dioxide removal using a water wash unit included in a chilled ammonia process. The method includes combining a CO 2  loaded liquid and a wash water liquid to form a CO 2  enriched wash water liquid that is then brought into contact with the combustion flue gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a divisional application ofU.S. application Ser. No. 13/357,963 filed Jan. 25, 2012 the contents ofwhich are hereby incorporated in its entirety.

TECHNICAL FIELD

The present invention relates to a method for treating a combustion fluegas. More specifically it relates to capturing ammonia in a chilledammonia process (CAP).

BACKGROUND

Liquid solutions comprising amine compounds or aqueous ammonia solutionsare commonly used as solvents in processes used for industrialseparation of acidic components such as H₂S, CO₂, COS and/or mercaptansfrom gas streams such as flue gas, natural gas, synthetic gas or othergas streams mainly containing nitrogen, oxygen, hydrogen, carbonmonoxide and/or methane. The acidic components are often absorbed in thesolvent in an absorption process or scrubbing process. After “scrubbing”of said acidic components by said solutions, contaminants, such astraces of ammonia, have to be removed from the gas stream in a separateprocess step.

The most commonly used process for this purpose is a wash or scrubbingstep of the contaminants. In such a wash water step, the gas stream isscrubbed with water in a suitable contacting device. Typically, thewater used to scrub the gas stream is either fresh water or very low NH₃content water obtained from a stripping process related to the treatmentof the gas stream. After the gas stream is scrubbed with water, thewater is 1) sent back to the stripping unit from which it was obtainedor 2) simply mixed with the solution used in the main scrubbing process.

There are methods known wherein the efficiency of the system and methodsare improved. In WO 2009/138363 it is disclosed a method for removal ofcontaminants from a gas stream by contacting the gas stream with CO₂containing liquid. The methods are said to be applicable forcontaminants like ammonia, where the emission of the contaminants isreduced. Also in U.S. Pat. No. 5,378,442 there is described a method tocontact CO₂ containing liquid for recovering of ammonia present in thecombustion exhaust gas.

Regeneration of used wash liquids in the scrubbing process, for examplein a stripping unit, is generally energy intensive and by that anexpensive process. Therefore, there is a constant need for processesthat improve wash efficiency and/or reduce wash liquid consumption.Regeneration of used wash liquids may be accomplished via strippingwhere a particular component is stripped from a wash liquid toregenerate the wash liquid.

SUMMARY

It is an object of the present invention to improve the efficiency of awash/scrubbing step in a gas purification process, more specifically, toimprove the capture and recovery of ammonia from a treated combustiongas in an absorber system.

The improved method and system for capturing ammonia in a chilledammonia process (CAP) according to various aspects described herein,ultimately allows a reduction in the concentration of ammonia exitingthe wash/scrubbing step and thus increases the quantity of recycleammonia back the absorber system. This helps to retain the concentrationof ammonia in solution in the absorber system and also to preventexcessive ammonia losses.

Reducing ammonia emission in the treated flue gas flowing from the waterwash unit supports retention of the ammonia in the chilled ammoniaprocess. It will also reduce the amount of sulfuric acid needed toneutralize ammonia when reheating the treated flue gas in a downstreamprocess.

According to aspects illustrated herein, there is provided a method forcapturing ammonia present in combustion flue gas having been subjectedto carbon dioxide removal in a water wash unit included in a chilledammonia process, comprising the steps of:

-   -   providing CO₂ loaded liquid comprising CO₂ dissolved in the        liquid;    -   providing wash water liquid;    -   combining the CO₂ loaded liquid with the wash water liquid to        form CO₂ enriched wash water liquid before the liquid is added        to the water wash unit to suppress the equilibrium vapor        pressure of NH₃ present over the surface of the CO₂ enriched        wash water liquid; and    -   bringing said combustion flue gas into contact with said CO₂        enriched wash water liquid by adding the CO₂ enriched wash water        liquid to said water wash unit.

The CO₂ loaded liquid from, for example, a CO₂ cooler is continuouslyadded to the wash water to maintain a low ammonia partial pressure Theamount of said liquid can be adjusted, reduced or increased, based onammonia emissions from the water wash system, and the required ammoniapartial pressure in the solution, in order to meet washing requirements.

According to some embodiments of the method, the concentration ofammonia in the wash water may be in the range of 0.0005-3 mol/liter. Ina water wash unit with a top stage and a lower stage, the concentrationof ammonia may, for example, be about 0.005 to 0.2 mol/l in the topstage, and about 0.5 to 3 mol/l in the lower stage. This concentrationcovers the range for both lean wash water and wash water mixed with theCO₂ loaded liquid. By operating with this concentration of ammonia inthe wash water, the vapor pressure of the ammonia can be kept at lowlevel, e.g., low enough to wash ammonia in the gas phase down to lessthan 200 ppm. In general, ammonia capture can be improved (and thepartial pressure of NH₃ kept low) by lowering the concentration of NH₃in the wash water solution, by lowering the operating temperature of thewash liquid and/or chemically depressing the partial pressure of ammoniavia the mixing of CO₂ loaded liquid streams. As long as the partialpressure of CO₂ over the said liquid is high, and solids are not formed,the concentration of NH₃ is of less importance.

According to some embodiments of the method, the ratio of moles ofammonia (NH₃) to moles of carbon dioxide (CO₂) (the R value) for the CO₂enriched wash water liquid is kept at about 0.05 to 10, preferably atabout 0.1 to 5, more preferably at about 1.

According to some embodiments of the method, the concentration ofammonia in the wash water is in the range of 0.0005-3 mol/liter,preferably in the range of 0.05-2 mol/liter, and a partial pressure ofCO₂ in the liquid phase between 1 and 20 bar.

According to some embodiments of the method, the wash water liquid usedfor ammonia removal comprises about 0.0005 mol/liter to 0.2 mol/literammonia (NH₃) before it is combined with the CO₂ loaded liquid.

According to some embodiments of the method, the operating temperatureof the wash water unit is about 1° C. to about 10° C.; preferably about5° C.

By performing the method for recapturing ammonia in these specifiedtemperature ranges the vapor pressure of ammonia may be kept low. Anyrefrigerant can be considered as working medium as long as theseoperating temperatures can be achieved. Suitable refrigerants may bepropane, propylene as well as ammonia.

According to some embodiments of the method, the ratio of moles ofammonia (NH₃) to moles of carbon dioxide (CO₂), also denoted as the Rvalue, is kept at about 0.05 to 10 for the CO₂ enriched wash waterliquid, preferably at about 0.1 to 5, more preferably about 1 to 4. Thelower R value of the water wash liquid the better results of the ammoniacapture.

According to aspects illustrated herein, there is provided a gaspurification system for capturing ammonia (NH₃) from combustion flue gasby bringing said gas into contact with CO₂ enriched wash water liquidcontaining dissolved carbon dioxide CO₂ in liquid form wherein thesystem comprises:

-   -   a water wash unit for capturing ammonia NH₃,    -   one or more wash water liquid ducts for recirculating wash water        liquid;    -   one or more units generating CO₂ loaded liquid;    -   a CO₂ loaded liquid duct transporting the CO₂ loaded liquid to        the wash water liquid duct from the one or more units for        generating CO₂ loaded liquid to suppress the equilibrium vapor        pressure of NH₃ over the wash water liquid; and    -   one or more CO₂ enriched wash water liquid ducts transporting        the CO₂ enriched wash water liquid resulting after integrating        the CO₂ loaded liquid and the wash water liquid to the water        wash unit for bringing the CO₂ enriched wash water liquid into        contact with the combustion flue gas.

According to some embodiments of the gas purification system, the unitsfor generating CO₂ loaded liquid is a CO₂ product cooler and/or a CO₂compressor system, working separately or together to generate CO₂ loadedliquid.

According to aspects illustrated herein, there is provided a gaspurification system for capturing ammonia (NH₃) from combustion exhaustgas by a wash water unit comprising at least one packed bed section,preferably two or more packed bed sections.

The water wash unit may be a suitable container, like a column. Thepacked bed may be selected to provide a sufficient mass transfer of thecomponents present in the water wash unit, thus to absorb the NH₃ fromthe combustion exhaust gas. The water wash unit may comprise one or morepacked beds, being the same or different, and arranged in differentways.

According to some embodiments of the gas purification system the CO₂enriched wash water liquid is introduced to the bottom section of thewash water unit by the CO₂ enriched wash water liquid duct.

The integration of CO₂ loaded liquid from the CO₂ product cooler and/orthe CO₂ product compressor can be introduced to either water wash topsection or water wash bottom section or in some cases in both sectionsof the water wash unit. Preferably it should be introduced in the topsection to achieve better performance.

According to some embodiments of the gas purification system, the waterliquid being subjected to ammonia capturing comprises less than 0.2mol/l ammonia (NH₃).

According to some embodiments of the gas purification system describedabove, water wash unit is operated at a temperature of about 1° C. toabout 10° C.; preferably about 5° C. The operating temperature of thesystem is dependent on the particular refrigerant used in the system.Suitable refrigerants may be propane, propylene, as well as ammonia.

According to some embodiments of the gas purification system, carbondioxide CO₂ in liquid form is reintroduced into the wash water liquidafter separation and liquefaction in a CO₂ product cooler unit.

According to some embodiments of the gas purification system, the carbondioxide CO₂ in liquid form is reintroduced into the wash water streamafter separation and liquefaction in a CO₂ product cooler unit forming aCO₂ cooler CO₂ loaded liquid.

According to some embodiments of the gas purification system, the carbondioxide CO₂ in liquid form is reintroduced into the wash water streamafter separation and liquefaction in a CO₂ compressor system forming aninterstage cooler CO₂ rich condensate.

According to some embodiments of the gas purification system, the carbondioxide CO₂ in liquid form is reintroduced into the water wash unitafter separation and liquefaction in a CO₂ product cooler unit incombination with a CO₂ compressor system.

The term “wash water”, as used herein, refers generally to an aqueousmedium used for removal of contaminants from a gas stream by bringingsaid gas stream into contact with said wash water, resulting in theabsorption of contaminants from said gas stream into said wash water.The wash water containing the absorbed contaminants is generallyrecycled, e.g., in a stripping unit, where the contaminants may beconcentrated for incineration or purification and reuse. In other words,the economics of the water wash step are dictated by the amount of washwater needed to reach the required removal levels of trace contaminants.The amount of wash water needed to properly scrub the gas stream isdictated by the absorption capacity of the water for the respectivetrace contaminants, i.e. the vapor/liquid equilibrium between thecontaminant in the gas phase and in the water phase.

Alternatively, the improved absorption capacity of the wash water may beused to further reduce the amount of contaminants present in the gasstream leaving the water wash step, without increasing wash waterconsumption. In other words, emissions can be reduced without acorresponding increase in costs due to increased water and energyconsumption.

The use of liquid CO₂ to improve the absorption capacity of wash wateris further advantageous because, e.g., i) CO₂ is odorless and relativelynon-toxic, ii) any CO₂ remaining in the wash water after use may easilybe removed during the regeneration of the wash water, and iii) CO₂ may,in at least some embodiments of the present invention, be readilyavailable as a product from another process step.

Alkaline compounds are often used in absorption processes for removal ofacidic gases, such as CO₂, H₂S from gas streams. Ammonia is one exampleof such alkaline compound, and the chilled ammonia process (CAP) is amethod for this. The gas purification method of the present invention isefficient for the removal of ammonia contaminating the gas stream fromuse in the chilled ammonia process. By the invention, a gas purificationsystem for the improved method is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram generally depicting an embodiment of an ammoniabased gas purification system according to the present invention.

FIG. 2 is a flow diagram generally depicting a known ammonia based gaspurification system (prior art).

DETAILED DESCRIPTION

Specific embodiments of gas purification systems of the prior art and ofthe present invention are described in detail hereinbelow with referenceto the drawings.

FIG. 1 is a schematic representation of an embodiment of an ammoniabased gas purification system 101 according to the present invention.The gas purification system 101 comprises a water wash unit 102 arrangedto allow contact between a gas stream to be purified and one or morewash liquids.

In accordance with one embodiment, the water wash unit 102 is arrangedfor cleaning a flue gas that has passed through a CO₂ absorber 140 of achilled ammonia process. The chilled ammonia process is, as such,described in, for example, WO 2006/022885 (Eli GAL). Hence, the CO₂absorber 140 may, for example, be arranged for capturing CO₂ from a fluegas of, for example, a power plant, an industrial plant, a wasteincineration plant or a metallurgical plant, in accordance with thechilled ammonia process. In the chilled ammonia process CO₂ is capturedin an ammoniated solution in the absorber 140, and the ammoniatedsolution is regenerated in a regenerator unit 142. Such regenerationinvolves heating the ammoniated solution to cause a release of CO₂. Forreasons of maintaining clarity of illustration FIG. 1 does notillustrate the flows of ammoniated solution between the CO₂ absorber 140and the regenerator unit 142, or the flow of flue gas through theabsorber 140.

Flue gas that has passed through the CO₂ absorber 140 for carbon dioxidecapture contains ammonia and is forwarded to water wash unit 102 via aduct 107 a for washing, as will be described in more detail hereinafter.

CO₂ product that is released as an effect of the heating of theammoniated solution in the regenerator unit 142 is forwarded via afluidly connected duct 142 a from regenerator unit 142 to a CO₂ productcooler unit 120. The CO₂ product cooler unit 120 purifies the CO₂product forwarded from regenerator unit 142 by capturing ammonia andcondensing water vapor from the CO₂ product. A liquid that containswater is circulated, via fluidly connected loop duct 121, in the CO₂product cooler unit 120. The liquid circulated in loop duct 121 iscooled in heat exchanger 121 a to cause condensation of water vapor fromthe CO₂ product. The liquid circulating in loop duct 121 of CO₂ productcooler unit 120 will capture ammonia and also some CO₂ from the CO₂product of the regenerator unit 142. Hence, the liquid circulating inloop duct 121 will contain some dissolved ammonia, and some dissolvedCO₂.

As will be described in more detail hereinafter, regenerated wash water,having a reduced content of ammonia, is forwarded to CO₂ product coolerunit 120 via duct 111, and a portion of the liquid circulated in CO₂product cooler unit 120 is forwarded from the unit 120 via duct 122fluidly connected to loop duct 121.

The water wash unit 102 is a mass transfer unit, which may comprise masstransfer enhancing arrangements, for example the water wash unit 102 maycomprise a column with a packed bed wherein the packing material isselected to optimize the mass transfer in the unit 102. The packingmaterial may be selected from many different suitable and commerciallyavailable packing materials. Also, the water wash unit 102 may bearranged to comprise one, two or more stages of washing, wherein thematerial forming the packed bed in each stage may be the same ordifferent, and the arrangements, such as, for example, random orstructured packaging, may be the same or different to optimizeparameters such as surface area, flow pattern, mass flow, etc. Theliquid flow through the unit 102 may also be arranged differentlybetween the different stages, to optimize the system and/or masstransfer. For example, the liquid flow may be in counter current mode,with the liquid flowing in the opposite direction of the gas, with thegas flowing vertically upwards and the liquid flowing verticallydownwards, or in co-current mode, with both the liquid and the gasflowing vertically down-wards. Furthermore, the liquid could either bearranged, for each of the stages, in a circulation mode, with the liquidbeing recirculated several times in the stage before being removedtherefrom, or in a once through arrangement, in which the liquid passesonce through the stage and is then removed therefrom.

In the specific embodiment of FIG. 1, the water wash unit is a waterwash unit 102 that comprises a two stage wash system having sectionswith different packing. The bottom section 103, i.e., the lower part ofthe water wash unit 102, comprises structured packing and is operated incounter current mode and with circulation mode with respect to theliquid solution, and with once through mode with respect to the fluegas. The top section 104, i.e., the second section of the water washunit 102, comprises random packing, and is operated in counter currentmode with once through water flow and once through flue gas flow. Fluegas to be cleaned enters the water wash unit 102 via duct 107 a. Cleanedflue gas leaves the water wash unit 102 via duct 107 b.

The used wash liquid leaving the water wash unit 102 contains absorbedammonia and leaves the water wash unit 102 via fluidly connected duct108. The used wash liquid may be at least partly recirculated andreintroduced to the water wash unit 102 and its lower part 103 viafluidly connected duct 105.

An option of the invention is that a portion of CO₂ may be introduced tothe wash liquid in duct 105, via fluidly connected duct 125, and CO₂containing wash liquid is thus introduced to the water wash unit 102 atthe bottom (first) section 103 of the unit 102. In combination with, oras alternative to, introducing a portion of CO₂ to the wash liquid induct 105, and as will also be described in more detail hereinafter, aportion of CO₂ may be introduced to the wash liquid in duct 106, viafluidly connected duct 122, and CO₂ containing wash liquid is thusintroduced to the water wash unit 102 at the upper (second) section 104of the unit 102.

The liquid introduced to the water wash unit 102, via duct 105 and/orduct 106, is denoted ‘CO₂ enriched wash water liquid’, which is the washwater resulting after the mixing of wash water liquid with the portionof CO₂. The portion of CO₂ may, as illustrated in FIG. 1, be CO₂ thathas been captured in the liquid of the CO₂ product cooler unit 120 fromthe CO₂ product forwarded from the regenerator 142. Such liquidcontaining a portion of CO₂ dissolved therein is forwarded from CO₂product cooler unit 120 to water wash unit 102 via fluidly connectedduct 122, and, optionally, via fluidly connected duct 125. The dissolvedCO₂ forwarded to the water wash unit 102 via duct 122, and optionallyduct 125, serves to improve the capture of ammonia in the water washunit 102 by reducing the vapor pressure of ammonia, as will be describedin more detail hereinafter.

The content of ammonia in the flue gas entering the water wash unit 102via duct 107 a may be about 5000-16000 ppm.

Flue gas with a reduced content of ammonia leaves the water wash unit102 via fluidly connected duct 107 b and is, for example, forwarded to adirect contact cooler (DCC) unit, not shown for reasons of maintainingclarity of illustration. The amount of ammonia in the flue gas leavingthe water wash unit 102 via duct 107 b may be about 0-500 ppm,preferably less than 200 ppm.

A portion, which may be referred to as “spent wash water”, of the washwater liquid leaving the water wash unit 102 via duct 108 may be fed toa heat exchanger 110 via fluidly connected duct 112. In the heatexchanger 110 the spent wash water coming from water wash unit 102 viaducts 108, 112 exchanges heat with a flow of regenerated wash watercoming from a stripper unit 130 via a fluidly connected duct 132. Thespent wash water coming from water wash unit 102 is, hence, forwarded toheat exchanger 110 via duct 112 and leaves heat exchanger 110 viafluidly connected duct 131. Fluidly connected duct 131 forwards thespent wash water to the stripper unit 130. Typically, the spent washwater forwarded to stripper unit 130 via fluidly connected duct 131 maycomprise ammonia in a concentration in the range of 0.5-3 mol/liter. Instripper unit 130 at least a portion of the content of ammonia of thespent wash water is removed, thereby generating, as will be described inmore detail hereinafter, a regenerated wash water, that leaves stripperunit 130 via the fluidly connected duct 132. Typically, the regeneratedwash water leaving stripper unit 130 via fluidly connected duct 132 maycomprise ammonia in a concentration in the range of 0.005-0.2 mol/liter.

The regenerated wash water is forwarded via duct 132 to the heatexchanger 110 in which the regenerated wash water is heat exchanged withthe spent wash water transported in ducts 112, 131. The regenerated washwater forwarded via duct 132 has a higher temperature than the spentwash water forwarded via duct 112. Hence, in heat exchanger 110 thespent wash water is heated before being forwarded, via fluidly connectedduct 131, to the stripper unit 130. Such reduces the amount of heat thatmust be supplied to stripper unit 130 to achieve the stripping ofammonia from the spent wash water. The regenerated wash water forwardedfrom stripper unit 130 via fluidly connected duct 132 is cooled in theheat exchanger 110 before being forwarded, via fluidly connected duct138 a, to fluidly connected duct 138 and further, optionally via heatexchanger 124, to the upper section 104 of the water wash unit 102, andvia fluidly connected duct 111 to the CO₂ product cooler unit 120.

Regenerated wash water is, hence, forwarded from the heat exchanger 110to the CO₂ product cooler unit 120 via fluidly connected ducts 138 a,111. The flow rate of the water flow to the CO₂ product cooler unit 120is typically about 5 l/min to 300 l/min, for example about 5 l/min to200 l/min. In the CO₂ product cooler unit 120, CO₂ containing water isrecirculated into the CO₂ cooler unit 120 by fluidly connected loop duct121. From the duct 121, a part of the CO₂ containing water is split andwater is transported to the water wash unit 102 via fluidly connectedduct 122, with a flow rate of about 5 l/min to 300 l/min. The liquidforwarded in duct 122 may also be denoted ‘CO₂ loaded liquid’, i.e.,liquid comprising the dissolved CO₂ and forwarded from the CO₂ coolerunit 120.

In one embodiment, the duct 122 is connected to the recycling loop, duct108 of the bottom section 103, via fluidly connected duct 125, whereinthe CO₂ containing water from the CO₂ product cooler unit 120 is mixedwith the water reintroduced via duct 105 after passing the heatexchanger 123, into the bottom, first section 103 of the water wash unit102.

In one embodiment of the invention, the duct 122 is fluidly connected tothe duct 138, wherein the CO₂ containing water is mixed with theregenerated wash water forwarded from the heat exchanger 110, andfurther forwarded via duct 106, to the water wash unit 102 and its topsection 104.

From the CO₂ product cooler unit 120 cooled CO₂ product is forwarded viaa duct 126 and an optional heat exchanger 127, to a CO₂ compressorsystem 150 generating a compressed CO₂ rich gas transported via fluidlyconnected duct 151 for further processing. The condensate, comprisingwater and CO₂, obtained in the CO₂ compressor system 150 as an effect ofintercooling between compression stages may be recycled to the gaspurification system 101 via fluidly connected duct 152. The liquid isherein denoted ‘CO₂ compressor interstage cooler CO₂ rich condensate’.The duct 152 is fluidly connected to the duct 122 and the ‘CO₂compressor interstage cooler CO₂ rich condensate’ is forwarded to thewater wash unit 102 as described above.

Optionally, in the gas purification system 101 the carbon dioxide CO₂ inliquid form is reintroduced into the water wash unit 102 via fluidlyconnected ducts 154 and 152 after separation and liquefaction in a CO₂product cooler unit 155, which may be a cryogenic unit for separatingcarbon dioxide from non-condensable gases, such as oxygen and nitrogen,such unit 155 being included in a high pressure CO₂ compressor system153.

In one embodiment, the CO₂ containing liquid is generated by combiningthe CO₂ cooler loaded wash water solution forwarded via duct 121 to duct122 and the CO₂ compressor interstage cooler CO₂ rich condensateforwarded via duct 152.

Optionally, the CO₂ containing water passes through heat exchanger units124 a, 124 b before entering the water wash unit 102 at a temperature ofabout 3 to about 7° C.

The heat exchanger unit 110 is fluidly connected to the stripper unit130, via fluidly connected ducts 131 and 132, wherein heat istransferred from the stripper bottom stream to the feed stream tominimize energy consumption in the stripper unit 130, as well as toprovide low temperature liquid to the water wash unit 102 to reducechiller load. For example, the stripper unit 130 may operate at atemperature of more than 120° C. and with a pressure of more than 20bar. The stripper unit 130 is heated by steam via fluidly connectedducts 136 and 137. In the stripper unit 130 ammonia is removed from thespent wash water coming from the water wash unit 102 via duct 131 andthe ammonia is, via fluidly connected duct 135, transferred to the CO₂absorber 140 for further treatment, such as capturing CO₂. The gascontaining ammonia and leaving the stripper unit 130 via a duct 133passes a condenser 134 on its way to the regenerator or absorber systemdepending on stripper operating pressure. A cooling liquid is forwardedto condenser 134 via a fluidly connected duct 134 a, and leaves thecondenser 134 via fluidly connected duct 134 b. The cooling liquidforwarded through condenser 134 via ducts 134 a, 134 b could be ofvarious origins. For example, the cooling liquid could be ammoniatedsolution forwarded from absorber 140 to regenerator unit 142 for beingregenerated therein. The cooling liquid of condenser 134 could also, forexample, be feed water for a boiler, or another cooling water availablein the plant. Vapor and liquid formed in the condenser 134 as an effectof the cooling of the gas leaving stripper unit 130 via duct 133 leavecondenser 134 via fluidly connected duct 133 a and are forwarded to avapor-liquid separator 135 a. In vapor-liquid separator 135 a gas andliquid are separated from each other. The liquid collected at the bottomof the vapor-liquid separator 135 a is returned, via fluidly connectedduct 135 b, to the stripper unit 130. In low-pressure stripperoperation, the overhead vapor stream is then transferred to the absorber140 via duct 135.

The systems described in detail above operate at a pressure of 20 bar.However, it shall be considered obvious that the systems are alsoapplicable for operation at a lower pressure, in an arrangement wherethe available parameters have been adjusted for achieving the NH₃capturing effect as is intended.

The gas entering the water wash unit 102 via the duct 107 a comprisestypically CO₂ in a concentration of 1.5-2.5% by volume.

The water wash unit 102 is typically operating at relatively high gasvelocities, such as in the range of 2-8 m/s, for example about 2.5 m/s.

By introducing a portion of CO₂, via a CO₂ containing liquid, into thewater wash unit 102, the mole ratio between the moles of ammonia to themoles of CO₂ may be lowered. Such lowering of the mole ratio between themoles of ammonia to the moles of CO₂ suppresses the equilibrium vaporpressure of NH₃ present over the surface of the CO₂ enriched wash waterliquid utilized in the water wash unit 102. In the top section 104 ofthe water wash unit 102, the concentration of ammonia of the CO₂enriched wash water liquid, forwarded via duct 106, may typically be0.005 to 0.2 mol/liter of NH₃. The ratio of moles of ammonia (NH₃) tomoles of carbon dioxide (CO₂) for the CO₂ enriched wash water liquidforwarded via duct 106 may typically be kept at about 0.05 to 10, andmore typically at about 0.05 to 2. In the bottom section 103 of thewater wash unit 102, the concentration of ammonia of the CO₂ enrichedwash water liquid, forwarded via duct 105, may be 0.5 to 3 mol/liter ofNH₃. The ratio of moles of ammonia (NH₃) to moles of carbon dioxide(CO₂) for the CO₂ enriched wash water liquid forwarded via duct 105 maytypically be kept at about 0.05 to 10, and more typically at about 0.5to 10.

The CO₂ product cooler unit 120 is also connected to the regeneratorunit 142, the regenerator unit 142 being arranged for regeneratingabsorption liquid that has been utilized in the absorber 140 forabsorbing CO₂ from, for example, flue gas in accordance with the chilledammonia process. Hence, the CO₂ product cooler unit 120 cools CO₂ thathas been released from the ammoniated solution in the regenerator unit142.

FIG. 2 is a schematic representation of a previously used gaspurification system 201 (prior art). The system comprises a water washunit 202 arranged to allow contact between a gas stream to be purifiedand one or more wash liquids.

The water wash unit 202 is represented in FIG. 2 and comprises a twostage wash system having sections with different packing. The bottomsection 203 in the lower part of the water wash unit 202 comprises astructured packed bed and is operated in circulation mode for thesolution and with once through mode for the flue gas. The top section204 in the top part of the water wash unit 202 comprises a random packedbed operating in counter current mode with once through water flow andonce through flue gas flow.

The used wash water liquid leaving the water wash unit 202 andcontaining absorbed ammonia leave the water wash unit via fluidlyconnected duct 208. The used wash water liquid may be recycled andreintroduced to the water wash unit 202 and its lower part via duct 205.

Flue gas having a reduced concentration of ammonia leaves the water washunit 202 via duct 207 and may be forwarded to a Direct Contact Cooler(DCC) unit, not illustrated for reasons of maintaining clarity ofillustration.

The wash water is fed to the heat exchanger unit 210 via duct 212. Wateris forwarded from the heat exchanger unit 210 to the CO₂ product coolerunit 220 via the duct 211.

Advantages of embodiments described hereinabove in connection with FIG.1 include:

Low concentration of NH₃ in the treated flue gas discharged from thewater wash unit 102;

Low consumption of acidifying components, like sulfuric acid, followingtreatment, such as in the direct contact cooling system (DCC) and directcontact heating (DCH) system;

Maintainability of the desired solution molarity in the systems forabsorption and regeneration;

Lower energy consumption of the stripper process;

Minimizing of amount of liquid required in the water wash unit 102 tocapture ammonia.

EXAMPLES Example 1 (Verification of Computer Model)

A computer model with a simulated water wash unit (A) in accordance withthe above described prior art system (FIG. 2) was compared with testresults (B) for a similar prior art system.

The simulation results showed 2.3% lower ammonia emission compared tothe test results, as shown in Table 1. Hence, the computer model wasconsidered a reasonable representation of a physical process and system.

TABLE 1 Comparison: Computer model to test result (prior art system)Inlet gas to water wash After bottom After top Case unit 202 stage 203stage 204 Unit A (model) NH₃ in 8897 2404 312 (in duct ppm gas 207) B(test) NH₃ in 8897 Not available 319 (in duct ppm gas 207)

Example 2 (Effect of Adding CO₂ Containing Liquid)

An introduction of CO₂ containing liquid from the CO₂ product cooler 120via duct 105 was made in a simulated water wash unit 102 (FIG. 1), thusCO₂ containing liquid was introduced to the bottom section 103 of thewater wash unit 102, and was compared to introducing CO₂ containingliquid from the CO₂ product cooler 120 via duct 106, thus to the topsection 104 of the water wash unit 102. The effect of the CO₂ containingliquid introduced in the water wash unit 102 is presented in table 2.The CO₂ containing liquid had a content of ammonia (mole/liter) of 0.54,and the mole ratio R was 1.05 (mole NH₃/mole CO₂), and the flow rate ofCO₂ containing liquid was measured to about 59 l/min at a flue gas flowin duct 107 b of about 40 800 kg/hour.

TABLE 2 Comparison: introduction of CO₂ containing liquid via duct 105,compared to introduction of CO₂ containing liquid via duct 106 Inlet gasto water wash After bottom After top Case unit 102 stage 103 stage 104Unit CO₂ via NH₃ in 8897 (in 1695 294 (in duct ppm duct 105 gas duct107a) 107b) CO₂ via NH₃ in 8897 (in 1736 171 (in duct ppm duct 106 gasduct 107a) 107b)

The results presented in Table 2 show that supply of CO₂ containingliquid via duct 106 to the top stage 104 of the water wash unit 102reduces the emission of ammonia by about 42% compared to introduction ofCO₂ containing liquid via duct 105 to bottom stage 103.

When comparing to the prior art results of Table 1, it is clear thatintroducing CO₂ containing liquid via duct 105 results in a reduction ofthe ammonia emission of about 6% (reduction from 312 to 294 ppm of NH₃),and that introducing CO₂ containing liquid via duct 106 results in areduction of the ammonia emission of about 45% (reduction from 312 to171 ppm of NH₃).

Example 3 (High Inlet Ammonia Concentration)

Simulations were made to test the ammonia emission at high ammoniaconcentration in the flue gas forwarded to the water wash unit, in theexample inlet ammonia is 16 000 ppm.

Comparative Example: Table 3 Illustrates the Simulated Result with thePrior Art Water Wash Unit 202 of FIG. 2

TABLE 3 Comparative example: Ammonia capture of prior art water washsystem 202. Inlet gas to water After bottom After top wash unit 202stage 203 stage 204 NH₃, (ppm) 15948 10157 2263Simulation of High Ammonia Concentration in Gas and Introduction of CO₂Containing Liquid Via Duct 105 or Via Duct 106:

The gas flow rate was kept at the same level as in Comparative example.CO₂ containing liquid from the CO₂ product cooler unit 120 was, in afirst simulation, added via the duct 105, to the bottom section 103 ofthe water wash unit 102. In a second simulation CO₂ containing liquidfrom the CO₂ product cooler unit 120 was added via the duct 106 to thetop section 104 of the water wash unit 102. The CO₂ containing liquidwas, in each simulation, added with a flow rate of 227 l/min at a fluegas flow in duct 107 b of about 40 800 kg/hour, concentration of ammoniawas kept at 1 mole/liter, and the mole ratio (mole NH₃/mole CO₂) was1.05.

The results achieved are shown in Table 4:

TABLE 4 Ammonia capture of water wash system 102, CO₂ introduced viaduct 105, or via duct 106. Inlet gas to water wash unit After bottomAfter top Case 102 stage 103 stage 104 CO₂ via duct NH₃, (ppm) 159484969 710 105 CO₂ via duct NH₃, ppm 15948 5605 159 106

As indicated above the emission of ammonia is reduced from about 2300ppm (table 3) as obtained for the prior art water wash system 202, toabout 710 ppm (table 4) with the water wash unit 102 with supply of CO₂to bottom section 103 via duct 105, and is reduced to about 160 ppm(table 4) by introducing the CO₂ containing liquid to the wash waterunit 102 at the top section 104 via the duct 106.

To summarize, a method for capturing ammonia present in combustion fluegas subjected to carbon dioxide removal, using a water wash unit (102)included in a chilled ammonia process, comprises:

-   -   providing CO₂ loaded liquid (122) comprising CO₂ dissolved in        the liquid;    -   providing wash water liquid (108, 138);    -   combining the CO₂ loaded liquid with the wash water liquid to        form CO₂ enriched wash water liquid (105, 106) before the liquid        is added said water wash unit (102); and    -   bringing said combustion flue gas into contact with said CO₂        enriched wash water liquid by adding the CO₂ enriched wash water        liquid to said water wash unit (102).

While the invention has been described with reference to variousexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

The invention claimed is:
 1. A method for capturing ammonia present incombustion flue gas subjected to CO₂ removal, using a water wash unitthat comprises at least first and second stages included in a chilledammonia process, comprising the steps of: providing CO₂ loaded liquidcomprising CO₂ dissolved in the liquid; providing wash water liquid;combining the CO₂ loaded liquid with the wash water liquid to form a CO₂enriched wash water liquid providing the CO₂ enriched wash water liquidat each of the first and second stages of the water wash unit; bringingthe combustion flue gas into contact with the CO₂ enriched wash waterliquid by adding the CO₂ enriched wash water liquid to said water wash;and forming a reduced ammonia flue gas stream and a used wash waterstream.
 2. The method according to claim 1, wherein the concentration ofammonia in the CO₂ enriched wash water liquid added to the first stageis 0.5 to 3 mol/liter.
 3. The method according to claim 1, wherein thewash water liquid comprises 0.0005 mol/l to 0.2 mol/l ammonia (NH₃)before it is combined with the CO₂ loaded liquid.
 4. The methodaccording to claim 1, wherein the wash water unit operates at 1° C. to10° C.
 5. The method according to claim 1, wherein the ratio of moles ofammonia (NH₃) to moles of carbon dioxide (CO₂) for the CO₂ enriched washwater liquid is kept at about 0.05 to
 10. 6. A method for capturingammonia present in combustion flue gas subjected to CO₂ removal, using awater wash unit that comprises at least first and second stages includedin a chilled ammonia process, comprising: providing CO₂ loaded liquidcomprising CO₂ dissolved in the liquid; providing wash water liquid;combining the CO₂ loaded liquid with the wash water liquid to form a CO₂enriched wash water liquid; providing the CO₂ enriched wash water liquidto the water wash unit; and bringing said combustion flue gas intocontact with said CO₂ enriched wash water liquid to form a cleaned fluegas stream and a used wash liquid.
 7. The method of claim 6, furthercomprising: providing a portion of the CO₂ enriched wash water liquid tothe water wash unit to each of the first and second stages.
 8. Themethod according to claim 5, wherein the ratio of moles of ammonia (NH₃)to moles of carbon dioxide (CO₂) for the CO₂ enriched wash water liquidis kept at about 0.1 to
 5. 9. The method according to claim 5 whereinthe ratio of moles of ammonia (NH₃) to moles of carbon dioxide (CO₂) forthe CO₂ enriched wash water liquid is kept at about 1 to
 4. 10. A methodfor capturing ammonia present in combustion flue gas subjected to CO₂removal, using a water wash unit that comprises at least first andsecond stages, the method comprising: providing a CO₂ loaded liquidcomprising CO₂ dissolved in the liquid; providing wash water liquid;combining the CO₂ loaded liquid with the wash water liquid to form CO₂enriched wash water liquid; providing the CO₂ enriched wash water liquidto the water wash unit to each of the at least first and second stages;bringing said combustion flue gas into contact with said CO₂ enrichedwash water liquid to form a reduced ammonia flue gas stream and a usedwash water stream; wherein the concentration of ammonia in the CO₂enriched wash water liquid added to the first stage is 0.5 to 3mol/liter; and wherein the concentration of ammonia in the CO₂ enrichedwash water liquid added to the second stage is 0.005 to 0.2 mol/liter.11. The method according to claim 10, wherein the wash water liquidcomprises 0.0005 mol/l to 0.2 mol/l ammonia (NH₃) before it is combinedwith the CO₂ loaded liquid.
 12. The method according to claim 10,wherein the wash water unit operates at 1° C. to 10° C.
 13. The methodaccording to claim 10, wherein the ratio of moles of ammonia to moles ofcarbon dioxide for the CO₂ enriched wash water liquid is kept at about0.05 to
 10. 14. The method according to claim 1, wherein theconcentration of ammonia in the CO₂ enriched wash water liquid added tothe second stage is 0.005 to 0.2 mol/liter.
 15. The method according toclaim 3, wherein the concentration of ammonia in the wash water enteringthe second stage is lower than the concentration of ammonia entering thefirst stage.